President Trump reversed himself on one of the key foreign policy issues of his presidency on Tuesday, abandoning his insistence that Ukraine give up land to strike a peace deal with Russia and instead declaring that Ukraine, with the support of Europe, was “in a position to fight and WIN all of Ukraine back in its original form.”

His turnabout on social media shortly after a meeting in New York with President Volodymyr Zelensky of Ukraine was a head-spinning pivot. After his three-hour meeting with Russia’s president, Vladimir V. Putin, in Alaska more than five weeks ago, he insisted that Mr. Zelensky would have to face reality and make a deal, giving up territory to its larger and stronger neighbor.

Mr. Trump provided no rationale for his stunning turnaround, though several European officials suspected that by distancing himself from the war, the president was washing his hands of a conflict that he once promised to solve in days or weeks. In his eight months in office, Mr. Trump has ricocheted from one position to another on Ukraine.

In February, he slammed Mr. Zelensky for insisting on American aid, yelling at him in the Oval Office, “You don’t have the cards.” In the spring, he cultivated Mr. Putin, exempting him from tariffs. This summer, he rolled out a red carpet for him in Alaska.

Now, he sounded as if he was siding anew with Ukraine while also taking something of a back seat, ending with the words: “I wish both Countries well.”